Heat the oven to 400°F. Wash the beets, wrap in foil, place on a sheet pan and roast in the oven until tender, 45 minutes to 1 hour. Let beets cool, then peel and grate using a box grater.
While allowing the beets to cool, make the Horseradish Sauce. In a small bowl, combine the yogurt, horseradish, lemon juice and olive oil. Season to taste with salt and pepper, and additional horseradish if you prefer it more spicy.
Heat 2 tablespoons butter over medium heat, add the onion and sauté until soft, about 5 minutes. Add the garlic and sauté until aromatic, about 1 more minute.
In a large bowl, combine the onion mixture with the beets, bread crumbs, eggs, cheese, and parsley. Add flour to bind. Season to taste with salt and pepper.
To test your canederli, bring a small pot of water to a boil. Take a small amount of the canederli mixture and form it into a ball about the size of a small strawberry. Place it in the boiling water. If the ball breaks apart, you need to add a bit more flour to the dough, and test it again. When it holds together as it cooks, eventually floating to the top when done, you are ready to form the rest of the mixture into canederli.
Form the mixture into 16 large round dumplings. Bring a large pot of water to a boil. Season with salt. Drop the canederli into the water and cook at a gentle simmer until they float to the top, 4 to 5 minutes. Remove from the water with a slotted spoon. Serve warm, with the horseradish sauce.